The present invention relates generally to an image display system for producing a color image by arranging a color shutter capable of time-divisionally switching a plurality of colors to be displayed, in front of a monochrome image display, and an image display method using the image display system.
As a method for displaying a color image, it is usually well carried out to synthesize images of red (R), green (G) and blue (B), which are three primary colors of light, or to divide the images. Such an image synthesizing method for displaying images is divided into a space sharing display for two-dimensionally arranging dots for R, G and B to spatially arrange an image, and a time sharing display for displaying R, G and B images in time series. Typically, color cathode ray tubes and liquid crystal displays use the space sharing display since R, B and G pixels are two-dimensionally arranged. Similarly, in the case of an image pickup, there are adopted methods for spatially arranging R, G and B color filters and for providing a color filter, which is capable of changing display colors in time series, in front of an image pickup element. A process for displaying an image will be briefly described below.
The time sharing display is achieved by quickly switching display colors on the whole display surface in synchronism with the display for R, G and B images by means of R, G and B color filters or the like. It is necessary for the time sharing display to switch the display for images at a higher speed than three times as high as that in the spatial sharing display. However, it is not necessary for the time sharing display to divide a pixel into parts for R, G and B images, so that it is possible to achieve a higher definition image. As a method for switching the display color, there is known a method for mechanically rotating a disc-like color filter which is divided into equal three parts and which is color-coded. As a method for electrically switching the display color, Bos, et al. proposes a technique in Japanese Patent Publication No. 4-49928, which discloses a so-called liquid crystal color shutter, which comprises two liquid crystal cells and color polarizing plates arranged on both sides thereof, for switching ON/OFF of the liquid crystal cells to control the plane of polarization for light to select the wavelength of light absorbed into the polarizing plates to display R, G and B. This liquid crystal color shutter has advantages in that there are no mechanical operations, the area of the color shutter can be equal to the area of the display screen to reduce required space, and so forth.
In the liquid crystal color shutter, the absorption axes of a plurality of color poling plates are perpendicular to each other. The two liquid crystal cells are turned ON and OFF to directly transmit the polarized light of an incident light or to rotate the polarized light by 90 degrees to transmit or absorb specific wavelength components, so that a desired display color can be obtained. One of conventionally proposed liquid crystal color shutters is a PI cell having a bend alignment. This can achieve a higher response speed than that of a TN (twisted nematic) cell, which is generally used as a liquid crystal display, i.e., a response time of about 2 ms.
On the other hand, in the case of the time sharing display, there is caused a so-called “color breakup” interference wherein the profile of a display image appears to be iridescent due to observer's blink, the movement of observer's eye, the movement of an object on a dynamic image, and so forth. In order to reduce the color breakup interference, it is desired to increase the switching speed for R, G and B to switch R, G and B as much as possible in a predetermined period of time such as one field period. For example, in the case of a triple speed display for displaying each of subfields R, G and B once in one field period, the display period for each color is 1/(60 Hz×3)=5.6 ms assuming that one field is 60 Hz. Here, one field period is defined as a period necessary for completing one color picture in spite of interlace or non-interlace display. For example, one field corresponds to 60 Hz in the case of an NTSC (National Television System Committee) color system for performing the interlace display.
However, if the frequency of subfields R, G and B displays in one field period is intended to increase in order to more reduce the color breakup interference, the response speed of the PI cell is not sufficient as shown in FIG. 1, so that it is required to provide a switching element having a higher response time. For example, when each of subfields R, G, B is displayed twice in a field of 60 Hz, the subfield period for each color is 1/(60×6)=2.8 ms. When 2 ms serving as a response time of the PI cell is subtracted from the display period for each color, it is possible to ensure only 800 μs, which is 28% of the whole display period for each color, as an appropriate display time.
As described above, in the case of the conventional time sharing type display system using the liquid crystal color shutter, there are problems in that the cell itself of the liquid crystal color shutter does not have a sufficient response time, so that a color breakup interference is caused.